Hello,
i've configured efa behind another antispam (I've just a external Ip address and I need to use another antispam for some domains and efa for each others).
So I've first antispam that send all emails for a efa managed domains but I'm getting this error:

have you tried to add your first antispam server into local networks in Postfix?
Webmin--->Server---->Postfix Mail Server---->General Options----->Local Networks
inside you should find localhost IP, and try to add, after, the public ip of first antispam server.